Photographic systems and processes having heat alterable spectral sensitivity

ABSTRACT

Reproduction systems having broader spectral sensitivity are produced wherein certain classes of dyes are added to photosensitive materials which, when activated, are capable of producing chemical reaction when in contact with image-forming agents to produce a visible image. The dyes are those which undergo a color change when heated to elevated temperatures and thus alter the spectral sensitivity of the photosensitive materials after heating. The dyes include styryl dyes substituted on the vinyl group by a nitrogen-containing heterocyclic, which in their unaltered state, themselves alter the sensitivity of the reproduction system. Improved processes using this improved reproduction system comprise selectively exposing the present systems to activating radiation after heating to the color transition temperature or alternatively first exposing and then heating to the said temperature. A preferred process is an add-on technique for addition of images to a reproduction system in which readable images are already present. The new reproduction systems of this invention have a built-in differential in spectral sensitivity by virtue of alteration of the dye by heating.

United States Patent [72] Inventor John R. Mnnhardt Nashua, NJil.

[21] Appl. No. 697,319

[22] Filed Jan. 12, 1968 [45] Patented Dec. 28, 1971 [73] Assignee ItelrCorporation Lexington, Mas.

[54] PHOTOGRAPHIC SYSTEMS AND PROCESSES HAVING HEAT ALTERABLE SPECTRALSENSITIVITY 32 Claims, No Drawings 96/1 R, 9611.5, 96/1 .7, 96/l.8,96/102, 96/48, 96/88, 96/1.6, 260/576, 252/501, 250/65 R 853,880 11/1960GreatBritain 944,362 12/1963 GreatBritain v1,091,715 11/1967GreatBritain Primary Examiner-George F. Lesmes Assistant Examiner-JohnC. Cooper Attorneys-Homer 0. Blair, Robert L. Nathans and W. GaryGoodson ABSTRACT: Reproduction systems having broader spectralsensitivity are produced wherein certain classes of dyes are added tophotosensitive materials which, when activated, are capable of producingchemical reaction when in contact with image-forming agents to produce avisible image. The dyes are those which undergo a color change whenheated to elevated temperatures and thus alter the spectral sensitivityof the photosensitive materials after heating. The dyes include styryldyes substituted on the vinyl group by a nitrogen-containingheterocyclic, which in their unaltered state, themselves alter thesensitivity of the reproduction system.

Improved processes using this improved reproduction system compriseselectively exposing the present systems to activating radiation afterheating to the color transition temperature or alternatively firstexposing and then heating to the said temperature. A preferred processis an add-on technique for addition of images to a reproduction systemin which readable images are already present. The new reproductionsystems of this invention have a built-in differential in spectralsensitivity by virtue of alteration of the dye by heating.

PHOTOGRAPHIC SYSTEMS AND PROCESSES HAVING HEAT ALTERAIILE SPECTRALSENSITIVITY BACKGROUND OF THE INVENTION .1. Field of the Invention Thisinvention relates to the field of photographic reproduction systems and,more specifically, to a photographic system utilizing an improved copymedium comprising a dye-sensitized photosensitive material which, whenactivated by suitable means, is capable of producing a visible image.

2. Description of the Prior Art Data or image storage media comprisingradiation sensitive materials such as titanium dioxide are described indetail in U.S.- Pat. Nos. 3,152,903; 3,052,541; French Pat. Nos. 345,206and 1,245,215 and in commonly owned copending US. Applications Ser. No.199,211 filed May 14, 1962 in the names of Elliot Berman et al., nowabandoned. In the aforementioned U.S. Patent Application,radiationsensitive titanium dioxide functions as a photosensitivecomponent of the media and exposure of said media to activating meanssuch as radiant energy, electron beams or the like results in thestorage of a reversible latent image pattern therein. The reversiblelatent image pattern exists for a limited time during which said patterncan be converted to an irreversible form and read out visually bycontacting said pattern with a suitable image forming material, such asa chemical redox system. In the aforesaid US. and French Patents, theradiation-sensitive material is combined with at least one component ofan imageforming material prior to exposure to activating means. Forexample, US. Pat. No. 3,152,903 describes a photosensitive material suchas titanium dioxide in combination with a reducible metal ion suchassilver nitrate. This copy media is exposed to activating means andthen contacted with a reducing agent to produce a visible image. US.Pat. No. 3,152,903 also discloses a system wherein the photosensitivematerial is used in combination with both an oxidizing agent such assilver nitrate and a reducing agent, such as Metol (1,4-methylparaminophenol sulfate). Upon exposure to suitable activating means, avisible image is formed. One of the limitations of the above-mentioneddata or image storage systems is that they lack the photographic speedof systems such as silver halide.

Commonly owned copending U.S. Application Ser. No. 633,689 filed Apr.26, 1967 describes a method to expand the possible uses of thesephotographic systems described in the above-mentioned Patents andApplication, by increasing the photographic speed of these systems byuse of cyanine and hemicyanine dyes with the photosensitive materials,including styryl dyes as described therein.

SUMMARY OF THE INVENTION It has now been unexpectedly found that thespectral sensitivity of reproduction systems may be altered byincorporating into the reproduction system a dye which undergoes colorchange when heated to elevated temperatures, thus altering the spectralsensitivity of the system. The contemplated dyes includestyryl-substituted nitrogen-heterocyclic dyes, especially those of theformula: ACR ECR,,-C H, N R R in which substituent A is anitrogen-containing heterocyclic nucleus; and substituents R,, R R and Rare each selected from the group consisting of hydrogen, alkyl, aryl,aralkyl and alkaryl groups. The preferred dyes of this formula are thosein which the heterocyclic nucleus contains at least one nitrogenheteroatom and may include other hetero atoms such as oxygen and sulfursuch as those derived from thiazole, isoxazole, quinoline,benzimidazole, indole, benzothiazole, thiadiazine, or similar suchheterocyclics and the substituents R and R, are hydrogen while R and Rare lower alkyl, especially methyl or ethyl. 1n the said dyes, theheterocyclic nucleus is most commonly in the form of a salt such as aquaternary ammonium salt or acid addition salt by virtue of the nitrogenin the heterocyclic ring. As is appreciated by those in the art, theheterocyclic nucleus may be substituted for example by various groupssuch as hydrocarbon radicals as previously enumerated, or alkoxy groups,without affecting the dye property of the compound. a

The present invention provides image reproduction systems comprising theaforesaid dyes in the heat transformed color and a photosensitivematerial which, when activated, is capable of producing chemicalreaction when in contact with image-forming agents to produce a visibleimage.

It will be understood that the copy media of this invention mayoptionally include imaging material such as above described. Therefore,one of the preferred embodiments of this invention is where an oxidizingagent is present on the dyed photosensitive substrate of this inventionat the time of exposure. .Another preferred embodiment is wherein thedyed photosensitive substrate includes not only an oxidizing agent butadditionally, a reducing agent at the time of exposure to suitableactivating means.

DESCRIPTION OF PREFERRED EMBODIMENTS The photoconductor or photocatalystpreferred in this invention are metal-containing photoconductors. Apreferred group of such photosensitive materials are the inorganicmaterials such as compounds of a metal and a nonmetallic element ofgroup VIA of the periodic table* (*Periodic table from Langes HANDBOOKOF CHEMISTRY, 9th edition pp. 56-57, 1956.) such as oxides, such as zincoxide, titanium dioxide, zirconium dioxide, germanium dioxide, indiumtrioxide; metal sulfides such as cadmium sulfide (CdS), zinc sulfide(ZNS) and tin disulfide (SnS metal selenides such as cadmium selenide(CdSe). Metal oxides are especially preferred photoconductors of thisgroup. Titanium dioxide is a preferred metal oxide because of itsunexpectedly good results. Titanium dioxide having an average particlesize less than about 250 millimicrons and which has been treated in anoxidizing atmosphere at a temperature between about 200 C. and 950 C.for from about 0.5 hours to about 30 hours is especially preferred and,more especially, that titanium dioxide produced by high temperaturepyrolysis of titanium halide.

Also useful in this invention as photoconductors are certain fluorescentmaterials. Such materials include for example, compounds such as silveractivated zinc sulfide, and zinc activated zinc oxide.

While the exact mechanism by which this invention works is not known, itis believed that exposure of photoconductor or photocatalysts of thisinvention to activating means causes an electron or electrons to betransferred from the valence band of the photoconductor or photocatalystto the conductance band of the same or at least to some similar excitedstate whereby the electron is loosely held, thereby changing thephotoconductor from an inactive form to an active form. If the activeform of the photoconductor or photocatalyst is in the presence of anelectron accepting compound a transfer of electrons will take placebetween the photoconductor and the electron accepting compound, therebyreducing the electron accepting compound. Therefore a simple test whichmay be used to determine whether or not materials have a photoconductoror photocatalytic effect is to mix the material in question with anaqueous solution of silver nitrate. Little, if any, reaction should takeplace in the absence of light. The mixture is then subjected to light,at the same time that a control sample of an aqueous solution of silvernitrate alone is subjected to light, such as ultraviolet light. If themixture darkens faster than the silver nitrate alone, the material is aphotoconductor or photocatalyst.

It is evident that the gap between the valence and the conducting bandof a compound determines the energy needed to make electron transitions.The more energy needed, the higher the frequency to which thephotoconductor will respond. It is known to the art that it is possibleto reduce the band-gap for these compounds by adding a foreign compoundas an activator which either by virtue of its atomic dimensions or bypossessing a particular electronic forbidden zone structure or throughthe presence of traps as donor levels in the intermediate zone betweenthe valence and the conduction band stresses the electronicconfiguration of the photoconductive compound, thereby reducing itsband-gap and thus increasing its ability to release electrons to itsconduction band.

The phenomenon observed with respect to the increased speed of thephotosensitive material containing the dye, prior to permanent colorchange or subsequent to color change, is not necessarily understood.However, color change of the dye during heating is gradual over a rangeof temperature up to the permanent color transition temperature and,until permanent transition is generally found to be thermoreversible,since cooling the dye results in change back to the original color.Further, this thermoreversibility of color change and the rapidity withwhich color change occurs indicate that a simple physical change isresponsible for the color change and extension of spectral sensitivity.This behavior of the specified dyes cannot be explained by mechanisminvolving thermal decomposition of the short chain dyes followed byrecondensation as a longer chain dye.

The foregoing theoretical explanation is not intended to be binding onthe applicant but rather is offered to permit a better understanding ofthe invention.

Regardless of the theory, the present invention comprises the process ofaltering the spectral sensitivity of a reproduction system by contactingthe system with a dye which undergoes a color change at elevatedtemperatures and heating the system.

The operability of any dye in the present process can be routinelydetermined by simply coating a suitable photosensitive medium ashereindescribed and then heating the medium to elevated temperatures. Ifcolor change occurs, then the dye is suitable for the process. Forexample, in general, the preferred dyes undergo color change attemperatures in the vicinity of about 200 F. Usually evidence of colorchange manifests itself on first heating the medium, and the colorchange is usually thermoreversible up to the permanent color transitiontemperature.

Photosensitive media containing the preferred dyes of this invention aresensitive to light of maximum wavelength of about 500 to below 600millimicrons, but, after heating, the sensitivity is extended to lightof longer wavelength, e.g. extending into the region of red sensitivity.When the permanent color transition temperature is reached, the extendedsensitivity is made permanent.

Advantage of this extended sensitivity can be taken by utilization ofthe differential spectral sensitivity of the photosensitive medium. Forexample, where the initial dyecontaining medium is not sensitive to redlight prior to heat treatment but is sensitive thereafter, red light maybe used to register an image on the photosensitive medium and, afterregistration of the image, the medium is heated to obtain a latent imagewhen the medium is rendered red light-sensitive. Such flexibility inreproduction systems is fully appreciated by those skilled in the art.For example, with media which have the possible extended sensitivity ofthe present invention, the registration of an image on the medium mustbe followed by a long wait period to permit loss of the activationcreated by impingement on the photosensitive surface, e.g. by darkstorage, or alternatively, the medium must be deactivated before formingthe desired registered latent image. The present process obviates theneed for long wait period between registration and formation of thedesired registered latent image. With media which lack the means ofextending the spectral sensitivity of the present invention, a movablecolored filter or second light source must be provided as a source ofactinic light for latent image formation after the registration iscompleted. This aspect of the present invention is of particularimportance where it is desired to add additional information tophotosensitive media on which is already present visible information orimages, i.e. as add-on process. For example, in data storage systemsbased on photosensitive media, it is possible to add distinctinformation utilizing the add-on technique.

For instance, an information storage tape comprising titanium dioxidecan be provided with an area for "add-on" information. This add-on areacan already contain a dye as described herein, or the dye can be addedas needed to specified areas. The information to be added can then beregistered in the desired position by the method described and, afterregistration, the registered latent image formed by the mere expedientof heating the medium. The latent image is rendered visible by contactwith an image-producing agent, as desired.

The use of red light for registering and imaging on the mediumpresupposes the use of a dye which, at the color transition temperature,is converted to a color which is sensitive to red light. Particularlyeffective in this use are the dyes in which substituent A is athiadiazole nucleus or a thiazole nucleus.

It is intended that the add-on process also embraces the use of dyeswhich do not necessarily extend light sensitivity of the photosensitivemedium to the red region, since it follows that the initial registrationof image on the medium can be effected with visible light (notnecessarily red light) of wavelength to which the medium is notsensitive prior to color change by heating but to which the heatedmedium is sensitive. The use of red light and the specified dyes merelyrepresents the preferred form of the invention.

The dyes of this invention may be used in solution to treat thephotosensitive materials prior to their incorporation into a copymedium. These dyed photosensitive materials can then be deposited on asubstrate, or incorporated into a substrate such as a fibrous web ofpaper. Alternatively, the dye can be combined with the photoconductivematerials in the copy medium, as for example, by dispersion of the dyein the binder for the photosensitive material. In addition, it ispossible to dip-dye the photosensitive substrate by merely immersing asubstrate containing the photosensitive material into a solution of theparticular dye. In coating a transparent film such as cellulosetriacetate, particular problems arose when applying a dyedphotoconductor in a gelatin binder to a transparent plastic film such ascellulose triacetate. This problem was overcome by first forming anaqueous slurry of the photosensitive material with gelatin dissolvedtherein and then incorporating a solution of the particular dye desired.This slurry is then applied to the film substrate to form a uniformlydyed photosensitive substrate having the desired increased speed andother desirable properties ofthis invention.

The inert carrier sheet upon which the photoconductor and dye of thisinvention are deposited comprises any suitable backing of sufficientstrength and durability to satisfactorily serve as a reproductioncarrier. The carrier sheet may be in any form such as, for example,sheets, ribbons, rolls, etc. This sheet may be made of any suitablematerials such as wood, rag content paper, pulp paper, plastics such as,for example, polyethylene terephthalate (Mylar) and cellulose acetate,cloth, metallic foil and glass. The preferred form of the carrier sheetis a thin sheet which is flexible and durable.

It is also useful to use a binder agent to bind the dye of thisinvention and photosensitive materials to the carrier sheet. In general,these binders are translucent or transparent so as not to interfere withtransmission of light therethrough. Preferred binder materials areorganic materials such as resins. Examples of suitable resins arebutadiene-styrene copolymer, poly (alkyl acrylates) such as poly-(methylmethacrylate), polyamides, polyvinyl acetate, polyvinyl alcohol andpolyvinylpyrrolidone.

The photoconductor should be conditioned in the dark before exposure.Such conditioning is generally conducted from 1 to 24 hours. Afterconditioning, the photoconductor is not exposed to activating radiationprior to its exposure to activating radiation for recording an imagepattern.

The period of exposure will depend upon the intensity of the lightsource, the particular imaging material, particular photoconductor, thetype and amount of catalyst, if any, and like factors known in the art.In general, however, the exposure may vary from about 0.001 seconds toseveral minutes.

While this invention is concerned particularly with forming a negativeimage of a positive print, or vice versa, it will be understood that theinvention described therein is also applicable to such positiveprocesses as that described in commonly assigned copending U.S. Pat. No.3,414,410. in this process, a photosensitive material is uniformly dyesensitized and then exposed to an image pattern of activating radiationto desensitize the dye-sensitized medium to activating radiation inthose portions thereof which are struck by radiation during the initialexposure, and then subsequently this thus partially desensitized mediumis exposed to activating radiation to activate those as yet unexposedareas of said medium which correspond with opaque areas of the originalimage. By contacting with image-forming material, as described in theprior art, a positive visible image of the original positive isproduced.

It is also within the scope of this invention to heat imagewise the dyedphotosensitive copy medium of this invention and then expose uniformlyto radiation to which the heated areas are sensitive and to which theunheated areas are insensitive. In the alternative, the uniform exposurecan be with radiation to which the unheated areas of the copy medium aresensitive, but to which the heated areas are insensitive. The activatedareas of the copy medium in either case may" then be developed to form apermanent image.

Image-forming materials which are useful in this invention are thosesuch as described in U.S. Pat. No. 3,152,903 and in copendingapplication Ser. No. 199,2] 1. These image-forming materials includepreferably an oxidizing agent and a reducing agent. Such image-formingmaterials are often referred to in the art as physical developers. Theoxidizing agent is generally the image-forming component of theimage-forming material. However, this is not necessarily true. Eitherorganic or inorganic oxidizing agents may be employed as the oxidizingcomponent of the image-forming material. Preferred oxidizing agentscomprise the reducible metal ions having at least the oxidizing power ofcupr'ic ion and include such metal ions as Ag", Hg, Pb, Au, Pt, Ni, Sn",Pb, Cu'", and Cu. Other suitable oxidizing agents useful in thisinvention as components of an image-forming material are permanganate(MnO,-) ion, various leuco dye materials such as disclosed in copendingapplication Ser. No. 623,534 filed in the name of L. Case, and the like.Organic oxidizing agents include tetrazolium salts, such as tetrazoliumblue and red, and diphenyl carbazone, and genarcyl red 68 (methine dye).

The reducing agent components of the image-forming materials of thisinvention include organic compounds such as the oxalates, formates,substituted and unsubstituted hydroxylamine, and substituted andunsubstituted hydrazine, ascorbic acid, aminophenols, and the dihydricphenols. Also, polyvinylpyrrolidone, alkali and alkaline earth metaloxalates and formates are useful as reducing agents. Suitable reducingcompounds include hydroquinone or derivatives thereof, 0- andp-aminophenol, p-methylaminophenol sulfate, p-hydroxyphenyl, glycine,oand p-phenylene diamine, and l-phenyl-3- pyrazolidone.

Additionally, the image-forming materials or physical developers maycontain organic acids which can react with metal ions to form complexmetal anions. Further, the developers may contain outer complexingagents and the like to improve image formation and other propertiesfound to be desirable in this art.

Additional stabilizing and fixing steps such as known to the art mayalso be added to the processes of this invention in order to increasethe life and permanence of the final print.

The following examples further illustrate the invention.

EXAMPLE 1 A mixture of 4 parts by weight of titanium dioxide and 1 partby weight of an emulsion of polyvinyl alcohol resin containing about 50percent of solids in water is used to coat paper sheets.

A sheet of the coated paper is then dipped into a solution containing2-[p-(dimethylamino)stryryl]-4-methyl-thiazole methochloride in methanoland then dried. The medium contained 0.5 percent dye based on thetitanium dioxide. The sotreated medium is heated on a hotplate and atabout 200 F., the color of the coating (which is normally pumpkinorange) begins to change in hue to violet which becomes more intense asthe temperature rises. Cooling of the paper caused a reversion in hue tothe original color and this cycling of hue can be repeated over and overwithout apparent fatigue.

At a temperature of 500 F., the hue change becomes permanent with thepurple color no longer reverting to orange upon cooling. A spectrogramof the heated medium shows a strong sensitization extending into thevisible above 600 millimicrons. The original dye-sensitized paper showedsensitization extending into the visible to a maximum of less than 600millimicrons.

The heated medium is then exposed at 400 F. to an image using a redlight source and the latent image so formed is converted to a visibleimage by development using alcoholic silver nitrate, followed by Metol.

Alternatively, filled paper sheets containing about 20 percent ofphotoconductive ZnO and TiO pigment are prepared in conventional papermaking apparatus by addition of an aqueous slurry of the pigment to thebeater and utilized in the above described procedure.

Finely divided water-insoluble photoconductive pigments are also dyesensitized by contacting the pigments with dilute solutions ofsensitizing dyes. Excess dye solution is decanted, and the treatedpigments are dried. The pigments can suitably be deposited without abinder on a substrate such as glass, or can be incorporated into aplastic or the fibrous web of a paper, or can be dispersed in a bindersuch as polyvinyl alcohol and used to coat rigid or flexibleelectrically insulating or conducting substrates.

EXAMPLE 2 A photosensitive copy medium as in example 1 is preparedcontaining 0.5 percent dye based on the titanium dioxide, the dye being2-[p-(dimethylamino)styryl]-3,5-dimethylthiadiazolium nitrate. The colorchange observed above 200 F. is more intense than that observed with thedye of example 1, although the hue also changes to purple. Similarly,above 500 F., the hue change is irreversible on cooling.

The spectral sensitivity of the unheated medium determined by exposurein a wedge spectograph for 1 minute, ceases at about 580 millimicronswhereas, after heating to 500 F., the spectral sensitivity extends to620 millimicrons.

Thus, heating to 500 F. changes the medium from a greensensitiveorthochromatic medium to a green and red sensitive short panchromaticmedium. Heating to the intermediate temperatures between 200 and 400 F.renders the medium both orthochromatic and panchromatic sensitive andreversible in spectral sensitivity.

The medium is imaged by exposure to red light and the image developed toobtain a visible image as described in example 1.

EXAMPLE 3 This example illustrates the add-on feature of this invention.

A dyed photosensitive medium as described in example 2, prior toheating, is exposed using a tungsten light source to a pattern ofactivating radiation. The exposed medium is developed by dipping into asaturated solution of silver nitrate in methanol and then into asolution of 5 g. of Phenidone, 40 g. of citric acid monohydrate in oneliter of methanol. A visible negative image of the light pattern isobtained. After fixing, the medium is dark adapted and exposed to asecond image of red light to which the medium is not sensitive. Afterregistering the image on the medium, the medium is heated to 400 F. andthe latent image corresponding to the second quinoline ACR =CR,-C l-l NR R in which substituent A is a nitrogen heterocyclic nucleus; andsubstituents R,, R R and R, are each selected from the group consistingof hydrogen, alkyl, aralkyl, aryl and alkaryl groups; and heating to atleast the color transition temperature of the dye.

2. Method as in claim 2 wherein the substituent A is selected from thegroup consisting of a thiazole nucleus, isoxazole nucleus, quinolinenucleus, benzimidazole nucleus, indole nucleus, benzothiazole nucleusand thiadiazole nucleus, said dye being in the form of a quaternary saltor acid addition salt.

3. Method as in claim 2 wherein the heating is conducted at atemperature of at least about 200 F.

4. Method as in claim 2 wherein the dye is4-methyl-2-[pdimethylaminostyryl] thiazole methochloride.

5. Method as in claim 2 wherein the dye is 2-[p-(dimethylamino)styryl1-3,S-dimethylthiadiazolium nitrate.

6. Method as in claim 5 wherein the heating is conducted at atemperature of about 400 F.

7. A method of forming an image in a photosensitive medium comprising ametal-containing photoconductor that is not sensitive to red lightradiation and which has been contacted with a dye of the formula:

ACR =CR,-C l-l N R,R in which substituent A is a nitrogen-containingheterocyclic nucleus and substituents R R R and R are each selected fromthe group consisting of hydrogen, alkyl, aralkyl, aryl, and alkarylgroups, comprising the steps of:

a. exposing the medium to an image with red light;

b. heating the medium to at least the color transition temperature ofthe dye to form a latent image corresponding to the original image.

8. Method as in claim 7 wherein step b is conducted before step a.

9. Method as in claim 7 wherein step a is conducted simul' taneous withstep b.

10. Method as in claim 7 wherein the latent image is rendered visible bycontact with an image-forming agent which undergoes anoxidation/reduction type reaction upon contact with an exposed,activated photoconductor.

11. Method as in claim 10 wherein the image-forming agent comprises asolution of a metal ion which is at least as strong an oxidizing agentas copper ion.

12. A method as in claim 11 wherein the metal ion is silver ion.

13. Method as in claim 11 wherein the solution includes a reducing agentfor the metallic ion.

14. Method of adding an image to a photosensitive medium comprising ametal-containing photoconductor already containing a visible imagethereon which comprises the successive steps of:

a. Contacting the photoconductor with a dye of the formula: ACR;,=CR,-CH, N R R wherein substituent A is a nitrogen-containing heterocyclicnucleus and substituents R,, R R and R are each selected from the groupconsisting ofhydrogen, alkyl, aralkyl, aryl and alkaryl groups;

b. registering an image on the medium with red light;

c. heating the medium after registering the image to at least the colortransition temperature of the dye to form a latent image correspondingto the registered image.

15. Method as in claim 14 wherein the latent image is rendered visibleby contact with an image-forming agent which undergoes anoxidation/reduction type reaction upon contact with an exposed.activated photoconductor.

16. Method as in claim 15 wherein the image forming agent comprises asolution of a metal ion which is at least as strong an oxidizing agentas copper ion.

17. Method as in claim 16, wherein the metal ion is silver ion.

18. Method as in claim 16 wherein the solution includes a reducing agentfor the metallic ion.

19. Method as in claim 15 wherein the substituent A is selected from thegroup consisting of a thiazole nucleus, isoxazole nucleus, quinolinenucleus, benzimidazole nucleus, indole nucleus, benzothiazole nucleusand thiadiazole nucleus, said dye being in the form of a quaternary saltor acid additional salt.

20. Method as in claim 19 wherein the heating is conducted at atemperature of at least about 200 F.

21. Method as in claim 14 wherein the dye is 4-methyl-2-[p-(dimethylamino)styryl] thiazole methochloride.

22. Method as in claim 14 wherein the dye is 2-[p-(dimethylamino)styryl1-3,S-dimethylthiadiazolium nitrate.

23. Method as in claim 22 wherein the heating is conducted at atemperature of about 400 F.

24. A photosensitive medium comprised of a metal containingphotoconductor and a dye of the formula: ACR =CR C H, N R,R whereinsubstituent A is a nitrogen heterocyclic nucleus; and substituents R R Rand R are each selected from the group consisting of hydrogen, alkyl,aralkyl, aryl and alkaryl groups; the medium having been heated to atleast the color transition temperature of the dye.

25. Medium as in claim 25 wherein the photoconductor comprises titaniumdioxide.

26. Medium as in claim 24 wherein the photoconductor is titanium dioxideof an average particle size of 250 millimicrons or less and the dye is4-methyl-2-[p' (dimethylamino)styryl] thiazole methochloride.

27. Medium as in claim 25 wherein the photoconductor is titanium dioxideof an average particle size of 250 millimicrons or less and the dye is2-[p-(dimethylamino)styryl]- 3,S-dimethylthiadiazolium nitrate.

28. Medium as in claim 25 wherein the substituent A is selected from thegroup consisting of a thiazole nucleus, isoxazole nucleus, quinolinenucleus, benzimidazole nucleus, indole nucleus, benzothiazole nucleusand thiadiazole nucleus, said dye being in the form of a quaternary saltor acid additional salt.

29. Medium as in claim 25 wherein the dye is 4-methyl-2-[p-(dimethylamino)styryl] thiazole methochloride.

30. Medium as in claim 25 wherein the dye is 2-[p-(dimethylamino)styryl1-3,S-dimethylthiadiazolium nitrate.

31. A method of recording a latent image which comprises heatingimagewise a copy medium comprising a photosensitive metal containingsemiconductor and a sensitizing dye which undergoes a color change uponsuch heating and then exposing the copy medium to radiation which willselectively activate the heated portions of the copy medium and whereinthe dye is of the formula ACR =CR,-C H, N R,R in which substituent A isa nitrogen heterocyclic nucleus; and substituents R,, R R and R, areeach selected from the group consisting of hydrogen, alkyl, aralkyl,aryl and alkaryl groups; and heating to at least the color transitiontemperature of the dye.

32. A method as in claim 31 comprising additionally contacting withimage forming materials comprising a solution of metal ions to form apermanent, irreversible image in the activated portions of the copymedium.

2. Method as in claim 2 wherein the substituent A is selected from thegroup consisting of a thiazole nucleus, isoxazole nucleus, quinolinenucleus, benzimidazole nucleus, indole nucleus, benzothiazole nucleusand thiadiazole nucleus, said dye being in the form of a quaternary saltor acid addition salt.
 3. Method as in claim 2 wherein the heating isconducted at a temperature of at least about 200* F.
 4. Method as inclaim 2 wherein the dye is 4-methyl-2-(p-dimethylaminostyryl) thiazolemethochloride.
 5. Method as in claim 2 wherein the dye is2-(p-(dimethylamino)styryl)-3,5-dimethylthiadiazolium nitrate.
 6. Methodas in claim 5 wherein the heating is conducted at a temperature of about400* F.
 7. A method of forming an image in a photosensitive mediumcomprising a metal-containing photoconductor that is not sensitive tored light radiation and which has been contacted with a dye of theformula: A-CR3 CR4-C6H4 N R1R2 in which substituent A is anitrogen-containing heterocyclic nucleus and substituents R1, R2, R3,and R4 are each selected from the group consisting of hydrogen, alkyl,aralkyl, aryl and alkaryl groups, comprising the steps of: a. exposingthe medium to an image with red light; b. heating the medium to at leastthe color transition temperature of the dye to form a latent imagecorresponding to the original image.
 8. Method as in claim 7 whereinstep b is conducted before step a.
 9. Method as in claim 7 wherein stepa is conducted simultaneous with step b.
 10. Method as in claim 7wherein the latent image is rendered visible by contact with animage-forming agent which undergoes an oxidation/reduction type reactionupon contact with an exposed, activated photoconductor.
 11. Method as inclaim 10 wherein the image-forming agent comprises a solution of a metalion which is at least as strong an oxidizing agent as copper ion. 12.Method as in claim 11 wherein the metal ion is silver ion.
 13. Method asin claim 11 wherein the solution includes a reducing agent for themetallic ion.
 14. Method of adding an image to a photosensitive mediumcomprising a metal-containing photoconductor already containing avisible image thereon which comprises the successive steps of: a.Contacting the photoconductor with a dye of the formula: A-CR3 CR4-C6H4N R1R2 wherein substituent A is a nitrogen-containing heterocyclicnucleus and substituents R1, R2, R3, and R4 are each selected from thegroup consisting of hydrogen, alkyl, aralkyl, aryl and alkaryl groups;b. registering an image on the medium with red light; c. heating themedium after registering the image to at least the color transitiontemperature of the dye to form a latent image corresponding to theregistered image.
 15. Method as in claim 14 wherein the latent image isrendered visible by contact with an image-forming agent which undergoesan oxidation/reduction type reaction upon contact with an exposed,activated photoconductor.
 16. Method as in claim 15 wherein the imageforming agent comprises a solution of a metal ion which is at least asstrong an oxidizing agent as copper ion.
 17. Method as in claim 16wherein the metal ion is silver ion.
 18. Method as in claim 16 wheReinthe solution includes a reducing agent for the metallic ion.
 19. Methodas in claim 15 wherein the substituent A is selected from the groupconsisting of a thiazole nucleus, isoxazole nucleus, quinoline nucleus,benzimidazole nucleus, indole nucleus, benzothiazole nucleus andthiadiazole nucleus, said dye being in the form of a quaternary salt oracid additional salt.
 20. Method as in claim 19 wherein the heating isconducted at a temperature of at least about 200* F.
 21. Method as inclaim 14 wherein the dye is 4-methyl-2-(p-(dimethylamino)styryl)thiazole methochloride.
 22. Method as in claim 14 wherein the dye is2-(p-(dimethylamino)styryl)-3,5-dimethylthiadiazolium nitrate. 23.Method as in claim 22 wherein the heating is conducted at a temperatureof about 400* F.
 24. A photosensitive medium comprised of a metalcontaining photoconductor and a dye of the formula: A-CR3 CR4-C6H4 NR1R2 wherein substituent A is a nitrogen heterocyclic nucleus; andsubstituents R1, R2, R3 and R4 are each selected from the groupconsisting of hydrogen, alkyl, aralkyl, aryl and alkaryl groups; themedium having been heated to at least the color transition temperatureof the dye.
 25. Medium as in claim 25 wherein the photoconductorcomprises titanium dioxide.
 26. Medium as in claim 25 wherein thephotoconductor is titanium dioxide of an average particle size of 250millimicrons or less and the dye is 4-methyl-2-(p-(dimethylamino)styryl)thiazole methochloride.
 27. Medium as in claim 25 wherein thephotoconductor is titanium dioxide of an average particle size of 250millimicrons or less and the dye is2-(p-(dimethylamino)styryl)-3,5-dimethylthiadiazolium nitrate. 28.Medium as in claim 25 wherein the substituent A is selected from thegroup consisting of a thiazole nucleus, isoxazole nucleus, quinolinenucleus, benzimidazole nucleus, indole nucleus, benzothiazole nucleusand thiadiazole nucleus, said dye being in the form of a quaternary saltor acid additional salt.
 29. Medium as in claim 25 wherein the dye is4-methyl-2-(p-(dimethylamino)styryl) thiazole methochloride.
 30. Mediumas in claim 25 wherein the dye is2-(p-(dimethylamino)styryl)-3,5-dimethylthiadiazolium nitrate.
 31. Amethod of recording a latent image which comprises heating imagewise acopy medium comprising a photosensitive metal containing semiconductorand a sensitizing dye which undergoes a color change upon such heatingand then exposing the copy medium to radiation which will selectivelyactivate the heated portions of the copy medium and wherein the dye isof the formula A-CR3 CR4-C6H4 N R1R2 in which substituent A is anitrogen heterocyclic nucleus; and substituents R1, R2, R3 and R4 areeach selected from the group consisting of hydrogen, alkyl, aralkyl,aryl and alkaryl groups; and heating to at least the color transitiontemperature of the dye.
 32. A method as in claim 31 comprisingadditionally contacting with image forming materials comprising asolution of metal ions to form a permanent, irreversible image in theactivated portions of the copy medium.